1. Field of Invention
The invention relates to both acceleration transducers and seismic instrumentation in general and in particular to micro-machined accelerometers and seismometers (velocimeters) and their associated electronics.
2. Description of Prior Art
Conventional broadband seismometers determine the ground motion due to a seismic event by measuring the motion of a suspended proof mass. The most sensitive seismometers measure the displacement of the proof mass using a differential capacitive position transducer. This transducer determines the displacement between two fixed plates either side of a parallel moving plate attached to the suspended proof mass by a differential measurement of the parallel-plate capacitances between the fixed and moving plates. The sensitivity of such transducers increases as the nominal gap between the fixed and moving plates is reduced. The greater the sensitivity of the transducer, the less subsequent electronic gain is needed and consequently the less the contribution of the electronics noise to the total sensor self noise. However, the thermal noise of the seismometer increases as the gap is reduced due to the increased damping. The damping of the proof-mass motion due to the gas in the gap between the two plates results in a residual thermal motion of the proof mass even in the absence of an external acceleration. This so-called squeeze-film damping increases with the reduction of the nominal gap. Hence optimum performance is a compromise between increasing the sensitivity and reducing the damping and hence the thermal noise. An indication of the noise-floor limit of a spring-mass sensor due to the intrinsic background thermal motion of the sensor""s proof mass is the so-called MTQ product, where M is the proof mass, T is the period of the proof mass resonant frequency, and Q is the quality factor of the spring-mass system. The noise-floor limit of a sensor is inversely proportional to the square root of MTQ and hence for the lowest noise the MTQ product must be maximized. This implies a large mass (broadband seismometers have masses of the order of 100 grams), a low resonant frequency (again, up to several seconds in a broadband instrument), and finally a high quality factor, Q, which due to viscous and material damping is normally rather modest in a conventional broadband seismometer. For the large proof-masses used in conventional mechanical seismometers, in the range of tens to hundreds of grams, this compromise still results in instruments readily capable of resolving signals down to the terrestrial noise floor.
The relatively small gaps that need to be maintained for this performance require that conventional mechanical seismometers need to be operated in a closed-loop configuration, with an actuator centering the proof-mass plate between the two fixed plates. An added advantage of using feedback is the ability to shape the frequency response of the seismometer and produce a linear signal; parallel-plate capacitive transducers are inherently nonlinear.
A seismic acceleration signal can, in general, be decomposed into two parts: a steady-state, or xe2x80x9cDCxe2x80x9d, signal due to the Earth""s gravity, and a varying, or xe2x80x9cACxe2x80x9d, signal due to seismic activity. Only the latter is of interest. A major problem in the design of seismic sensors is the very large ratio between these two components; the seismic signal is often more than 160 dB smaller than the gravitational signal. When feedback is used in such circumstances, the actuator is unable to exert a large enough force to counteract the DC force and move the suspended proof mass to approximately a null position without severely degrading the sensor self noise; if a large feedback force were utilized to null the effect of gravity the noise generated by the actuator would dominate the instrument performance. Thus, seismometers employ various mechanical spring (also referred to as flexure) mechanisms to support the static proof mass in the desired orientation by overcoming the DC force, but these spring (flexural) mechanisms have a limited restoring range. Hence, tilt misalignment during deployment, which will result in a residual gravitational force on the proof mass moving the plate gap off the nominal null position, must be minimized, usually to less than a degree.
Conventional broadband seismometers produce a velocity output, preferable in terrestrial seismology since the background-seismic-velocity noise is evenly spread in its power spectrum and so such an output allows the greatest dynamic range of the seismic signal over the band of interest.
There are many limitations to conventional broadband seismometers:
Relatively large size (15 cmxc3x9715 cmxc3x9720 cm) for a three component device
Relatively heavy mass (several kilograms)
Requirement for accurate leveling either manually or using a complex automatic control system and mechanism
Requirement for a stable mounting surface that does not vary over time
Sensitivity of the suspension to temperature variation requiring a relatively well controlled thermal environment.
Relatively high electrical power consumption (1 to 3W)
Delicate suspensions requiring the mass to be mechanically locked before transportation.
High cost
Several attempts have been made to produce seismometers using MEMS technologies to overcome these disadvantages. Commercially, MEMS great advantage is in using high-definition processes developed by the semiconductor industry, originally for batch electronics fabrication, to machine mechanical structures at otherwise unavailable spatial resolution. The repeatability of the processes results in a large number of devices with well-characterized performance, while the batch processing results in low unit cost.
Maximizing MTQ has proved very problematical for silicon sensors and currently the state of the art sensor do not approach the desired performance. The first limitation is that mass requires volume, and the maximum practical size for the entire die is of the order of 2 cm square with a wafer thickness of 1 mm. A proof mass using most of this die volume would weigh a few grams at most.
The next limitation is the natural frequency of the spring-mass system, determined by the relative strength of the spring to the mass. As the natural frequency is reduced, the sag of the linear suspension under gravity determines the overall dimensions of an open-loop sensor. A 10-Hz suspension will sag by 2.5 mm under 1 g requiring sensor geometry large enough to accommodate this movement of the proof mass. No existing micromachined sensor uses such a low resonant frequency. Thus, the MT of the MTQ product is already a factor of 100 to 1000 less than the conventional broadband sensor.
The only factor left is the quality factor, Q, of the suspension. This is the area where efforts have been concentrated in realizing a practical silicon seismic sensor. Unfortunately, although silicon is an excellent mechanical material capable of sustaining Q""s of the order of more than 10,000, the practicalities of the device structure results in a degradation from these high values due to viscous gas damping. Currently the only approach to reducing this viscous damping is to seal the device in a very high vacuum and maintain this vacuum throughout the life of the product. This vacuum needs to be less than about one mTorr as the damping is independent of pressure until the residual gas in the cavity is rarified. This requirement greatly increases the difficultly and cost of making a silicon micro-seismometer. In addition, existing micromachined accelerometers with feedback use electrostatic actuation. This has the disadvantage of either producing an acceleration rather than the preferred velocity output or, in order to produce a velocity output, injecting excess noise due to the required active differentiator in the feedback path.
The present invention comprises an in-plane suspension geometry combined with a transverse periodic-sensing-array position transducer with open-loop DC and closed-loop AC operation to provide many improvements over the prior art. The use of a transverse motion in the mass eliminates squeeze-film damping allowing a high Q suspension to be developed without the need for the vacuum encapsulation of the prior art. The use of deep reactive-ion etching to form this structure provides for improvements over the prior art such as the creation of compact highly resilient suspension with a low resonant frequency, and a very good aspect ratio of the springs, minimizing unwanted out-of-plane motion. A large displacement of the proof mass can be accommodated to allow operation of a relatively low-resonant-frequency suspension in a large DC acceleration field, increasing the mechanical gain of the suspension and reducing the suspension noise. As the full output swing of the periodic sensing array is available for a displacement which is only a fraction of the full-scale displacement, a much larger gain is possible for the position transducer, reducing the transducer""s noise. The use of a feedback actuator of magnetic design, incorporating fixed magnets and planar coils on the surface of the proof mass, offers advantages over the prior art. The magnetic actuator provides much lower noise than an equivalent electrostatic actuator, and does not require high voltages. Feedback allows the micro-seismometer""s response to be flat across the frequency band of interest, in contrast to the very peaked open-loop response which would otherwise be obtained from such a high-Q suspension. When combined with the periodic sensing array above, the feedback need only operate over slightly more than one period of the position transducer, reducing the force required and hence both the power and noise of the actuator. Limit control electronics are incorporated in the feedback loop to allow for the transition between adjacent null points. Although the magnetic actuators used in this invention have been described in the literature this particular application is unique and novel in that the use is not actually to create motion for a positioning mechanism but to oppose it in a sensing system.
The sensor can be deployed at a random orientation and so operate without complex manual or automatic tilt-removal mechanisms, allowing measurement at sites where the mounting platform is not stable. Further the suspension is sufficiently rugged that no mass locking is required.
Finally, due to the small size of the proposed device it is possible to actively control its temperature using a Thermo-Electric-Cooler (TEC) a co-located temperature sensor and a control system. This technique allows operation with temperature variations of below 0.01C. using currently available technology, thus removing the requirements for external control of the thermal variations.